This is actually my first computer mod, I started this months ago before I had even heard of modding. I wanted a Media center computer to sit in my entertainment center that kind of fits in with stereo equipment. Unfortunately, I didn't start taking pics until halfway done, so bear with me. All I can really do at this point is show you what is done a describe it.
I took a very old computer (like a pentium 1 or something) and gutted it, leaving only the metal frame.
I then used some scrap red oak plywood to cover the sides and I was lucky enough to have a scrap solid oak plank for the front. These were sanded, screwed to the frame and painted.
I removed the CD rack and turned it sideways, then re-riveted it to the frame.
This is the under-side which is still footless, but I made a bevelled edge.
I had to cut some metal away for the power supply to fit.
Also had to cut the back panel off to orient the motherboard to fit, not much left of the original frame...